Intervertebral spacer device having a belleville washer with concentric grooves

ABSTRACT

An intervertebral spacer device may include first and second plates, each having inner and outer surfaces thereof, the plates may be disposed in a spaced apart relationship such that the inner surfaces face toward one another, and the outer surfaces face away from one another, the first plate including a retaining wall extending outwardly from the inner surface of the first plate; and a belleville washer, having narrow and wide ends thereof, may be disposed with said wide end contacting said inner surface of said first plate within said retaining wall, such that a compressive load applied to the outer surfaces of said plates is counteracted by said belleville washer, said belleville washer including a plurality of radially spaced concentric groove.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.10/833,736, filed on Apr. 28, 2004, now U.S. Pat. No. 7,771,477 which isa continuation of U.S. application Ser. No. 09/974,154, filed Oct. 11,2001, now U.S. Pat. No. 6,887,274, which is a continuation-in-part ofU.S. application Ser. No. 09/970,479, filed Oct. 4, 2001, now U.S. Pat.No. 6,669,730, which is a continuation-in-part of Ser. No. 09/968,046,filed Oct. 1, 2001, now abandoned, the disclosures of which are herebyincorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to simultaneously provide stabilization and continued flexibilityand proper anatomical motion, and more specifically to such a devicewhich utilizes a spirally slotted belleville washer, having radiallyspaced concentric grooves, as a restoring force generating element.

BACKGROUND OF TEE INVENTION

The bones and connective tissue of an adult human spinal column consistsof more than 20 discrete bones coupled sequentially to one another by atri-joint complex which consists of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than 20 bones are anatomically categorized as being membersof one of four classifications: cervical, thoracic, lumbar, or sacral.The cervical portion of the spine, which comprises the top of the spine,up to the base of the skull, includes the first 7 vertebrae. Theintermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic spine,which are in turn smaller than those of the lumbar region. The sacralregion connects laterally to the pelvis. While the sacral region is anintegral part of the spine, for the purposes of fusion surgeries and forthis disclosure, the word spine shall refer only to the cervical,thoracic, and lumbar regions.

The spinal column of bones is highly complex in that it includes overtwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complications,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumors,and degenerative wear are a few of the causes that can result in spinalpathologies for which surgical intervention may be necessary. A varietyof systems have been disclosed in the art which achieve immobilizationand/or fusion of adjacent bones by implanting artificial assemblies inor on the spinal column. The region of the back which needs to beimmobilized, as well as the individual variations in anatomy, determinethe appropriate surgical protocol and implantation assembly. Withrespect to the failure of the intervertebral disc, the interbody fusioncage has generated substantial interest because it can be implantedlaparoscopically into the anterior of the spine, thus reducing operatingroom time, patient recovery time, and scarification.

Referring now to FIGS. 1 and 2, in which a side perspective view of anintervertebral body cage and an anterior perspective view of a postimplantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 10 generally comprise tubular metal body 12 having an externalsurface threading 14. They are inserted transverse to the axis of thespine 16, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 2 the pair of cages 10 are inserted betweenthe fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages10 are generally inserted side by side with the external threading 14tapping into the lower surface of the vertebral bone above (L5), and theupper surface of the vertebral bone (S1) below. The cages 10 includeholes 18 through which the adjacent bones are to grow. Additionalmaterial, for example autogenous bone graft materials, may be insertedinto the hollow interior 20 of the cage 10 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 10.

These cages of the prior art have enjoyed medical success in promotingfusion and grossly approximating proper disc height. It is, however,important to note that the fusion of the adjacent bones is an incompletesolution to the underlying pathology as it does not cure the ailment,but rather simply masks the pathology under a stabilizing bridge ofbone. This bone fusion limits the overall flexibility of the spinalcolumn and artificially constrains the normal motion of the patient.This constraint can cause collateral injury to the patient's spine asadditional stresses of motion, normally borne by the now-fused joint,are transferred onto the nearby facet joints and intervertebral discs.It would therefore, be a considerable advance in the art to provide animplant assembly which does not promote fusion, but, rather, whichnearly completely mimics the biomechanical action of the natural disccartilage, thereby permitting continued normal motion and stressdistribution.

It is, therefore, an object of the present invention to provide a newand novel intervertebral spacer which stabilizes the spine withoutpromoting a bone fusion across the intervertebral space.

It is further an object of the present invention to provide an implantdevice which stabilizes the spine while still permitting normal motion.

It is further an object of the present invention to provide a device forimplantation into the intervertebral space which does not promote theabnormal distribution of biomechanical stresses on the patient's spine.

Other objects of the present invention not explicitly stated will be setforth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects of the invention are achieved by the presentinvention which is a flexible intervertebral spacer device comprising apair of spaced apart base plates, arranged in a substantially parallelplanar alignment (or slightly offset relative to one another inaccordance with proper lordotic angulation) and coupled to one anotherby means of a spring mechanism. In particular, this spring mechanismprovides a strong restoring force when a compressive load is applied tothe plates, and may also permit rotation of the two plates relative toone another. While there are a wide variety of embodiments contemplated,a preferred embodiment includes a belleville washer utilized as therestoring force providing element, the belleville washer being spirallyslotted and having radially spaced concentric grooves.

More particularly, as the assembly is to be positioned between thefacing surfaces of adjacent vertebral bodies, the base plates shouldhave substantially flat external surfaces which seat against theopposing bone surfaces. Inasmuch as these bone surfaces are oftenconcave, it is anticipated that the opposing plates may be convex inaccordance with the average topology of the spinal anatomy. In addition,the plates are to mate with the bone surfaces in such a way as to notrotate relative thereto. (The plates rotate relative to one another, butnot with respect to the bone surfaces to which they are each in contactwith.) In order to prevent rotation of a plate relative to the bone, theupper and lower plates can include a porous coating into which the boneof the vertebral body can grow. (Note that this limited fusion of thebone to the base plate does not extend across the intervertebral space.)

In some embodiments (not in the preferred embodiment), between the baseplates, on the exterior of the device, there is included acircumferential wall which is resilient and which simply preventsvessels and tissues from entering within the interior of the device.This resilient wall may comprise a porous fabric or a semi-impermeableelastomeric material. Suitable tissue compatible materials meeting thesimple mechanical requirements of flexibility and durability areprevalent in a number of medical fields including cardiovascularmedicine, wherein such materials are utilized for venous and arterialwall repair, or for use with artificial valve replacements.Alternatively, suitable plastic materials are utilized in the surgicalrepair of gross damage to muscles and organs. Still further materialsthat could be utilized herein may be found in the field of orthopedic inconjunction with ligament and tendon repair. It is anticipated thatfuture developments in this area will produce materials that arecompatible for use with this invention, the breadth of which shall notbe limited by the choice of such a material.

As introduced above, the internal structure of the present inventioncomprises a spring member, which provides a restoring force whencompressed. More particularly, it is desirable that the restoring forcesbe directed outward against the opposing plates, when a compressive loadis applied to the plates. In addition, in certain embodiments, it isnecessary that the restoring force providing subassembly notsubstantially interfere with the rotation of the opposing platesrelative to one another. In the preferred embodiment, the springsubassembly is configured to allow rotation of the plates relative toone another. In other embodiments, the spring subassembly can beconfigured to either allow rotation of the plates, or prevent rotationof the plates (through the tightening of a set screw as discussedbelow). As further mentioned above, the force restoring member comprisesat least one belleville washer.

Belleville washers are washers which are generally bowed in the radialdirection. Specifically, they have a radial convexity (i.e., the heightof the washer is not linearly related to the radial distance, but may,for example, be parabolic in shape). The restoring force of a bellevillewasher is proportional to the elastic properties of the material. Inaddition, the magnitude of the compressive load support and therestoring force provided by the belleville washer may be modified byproviding slots and/or grooves in the washer. In the preferredembodiment of the present invention, the belleville washer utilized asthe force restoring member is spirally slotted, with the slotsinitiating on the periphery of the washer and extending along arcs whichare generally radially inwardly directed a distance toward the center ofthe bowed disc, and has radially uniformly spaced concentric grooves ofuniform width and depth.

As a compressive load is applied to a belleville washer, the forces aredirected into a hoop stress which tends to radially expand the washer.This hoop stress is counterbalanced by the material strength of thewasher, and the strain of the material causes a deflection in the heightof the washer. Stated equivalently, a belleville washer responds to acompressive load by deflecting compressively, but provides a restoringforce which is proportional to the elastic modulus of the material in ahoop stressed condition. With slots and/or radially spaced concentricgrooves formed in the washer, it expands and restores itself far moreelastically than a solid washer.

In general, the belleville washer is one of the strongest configurationsfor a spring, and is highly suitable for use as a restoring forceproviding subassembly for use in an intervertebral spacer element whichmust endure considerable cyclical loading in an active human adult.

In the preferred embodiment of the present invention, a single modifiedbelleville washer, which is of the slotted variety and has radiallyspaced concentric grooves as described above, is utilized in conjunctionwith a ball-shaped post on which it is free to rotate through a range ofangles (thus permitting the plates to rotate relative to one anotherthrough a corresponding range of angles). More particularly, thisembodiment comprises a pair of spaced apart base plates, one of which issimply a disc shaped member (preferably shaped to match the end of anintervertebral disc) having an external face (having the porous coatingdiscussed above) and an internal face having an annular retaining wall(the purpose of which will be discussed below). The other of the platesis similarly shaped, having an exterior face with a porous coating, butfurther includes on its internal face a central post portion which risesout of the internal face at a nearly perpendicular angle. The top ofthis post portion includes a ball-shaped knob. The knob includes acentral threaded axial bore which receives a small set screw. Prior tothe insertion of the set screw, the ball-shaped head of the post candeflect radially inward (so that the ball-shaped knob contracts). Theinsertion of the set screw eliminates the capacity for this deflection.

As introduced above, a modified and spirally slotted belleville washerhaving radially spaced concentric grooves is mounted to this ball-shapedknob in such a way that it may rotate freely through a range of anglesequivalent to the fraction of normal human spine rotation (to mimicnormal disc rotation). The belleville washer of this design is modifiedby including an enlarged inner circumferential portion (at the center ofthe washer) which accommodates the ball-shaped portion of the post. Moreparticularly, the enlarged portion of the modified belleville washerincludes a curvate volume having a substantially constant radius ofcurvature which is also substantially equivalent to the radius of theball-shaped head of the post. The deflectability of the ball-shaped headof the post, prior to the insertion of the set screw, permits the headto be inserted into the interior volume at the center of the bellevillewasher. Subsequent introduction of the set screw into the axial bore ofthe post prevents the ball-shaped head from deflecting. Thereby, thewasher can be secured to the ball-shaped head so that it can rotatethereon through a range of proper lordotic angles (in some embodiments,a tightening of the set screw locks the washer on the ball-shaped headat one of the lordotic angles).

This assembly provides ample spring-like performance with respect toaxial compressive loads, as well as long cycle life to mimic the axialbiomechanical performance of the normal human intervertebral disc. Thespiral slots and radially spaced concentric grooves of the bellevillewasher allow the washer to expand radially as the slots and grooveswiden under the load, only to spring back into its undeflected shapeupon the unloading of the spring. As the washer compresses anddecompresses, the annual retaining wall maintains the wide end of thewasher within a prescribed boundary on the internal face of the baseplate which it contacts, and an annular retaining ring maintains thewide end of the washer against the internal face.

Finally, inasmuch as the human body has a tendency to produce fibroustissues in perceived voids, such as may be found within the interior ofthe present invention, and such fibrous tissues may interfere with thestable and/or predicted functioning of the device, some embodiments ofthe present invention (although not the preferred embodiment) will befilled with a highly resilient elastomeric material. The material itselfshould be highly biologically inert, and should not substantiallyinterfere with the restoring forces provided by the spring-likemechanisms therein. Suitable materials may include hydrophilic monomerssuch as are used in contact lenses. Alternative materials includesilicone jellies and collagens such as have been used in cosmeticapplications. As with the exterior circumferential wall, which wasdescribed above as having a variety of suitable alternative materials,it is anticipated that future research will produce alternatives to thematerials described herein, and that the future existence of suchmaterials which may be used in conjunction with the present inventionshall not limit the breadth thereof.

BRIEF DESCRIPTION OF TEE DRAWINGS

FIG. 1 is a side perspective view of an interbody fusion device of theprior art.

BRIEF FIG. 2 is a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devicesof the type shown in FIG. 1 have been implanted.

FIGS. 3 a and 3 b are side cross-section views of the upper and loweropposing plates of the preferred embodiment of the present invention.

FIGS. 4 a and 4 b are top and side cross-section view of a bellevillewasher having radially uniformly spaced concentric grooves of uniformwidth and depth and spiral slots, for use in a preferred embodiment ofthe present invention.

FIGS. 5 a-5 c are top and side cross-section views of a bellevillewasher having radially non-uniformly spaced concentric grooves ofvarying width and depth and spiral slots, for use in an alternateembodiment of the present invention.

FIG. 6 a is a top view of the upper plate of FIG. 3 a, with thebelleville washer of FIGS. 4 a and 4 b fitted within a retaining walland a retaining ring of the upper plate.

FIG. 6 b is a top view of the lower plate of FIG. 3 b.

FIG. 7 is a side cross-section view of the preferred embodiment of thepresent invention, which utilizes a belleville washer of the type shownin FIGS. 4 a and 4 b, showing the plates of FIGS. 6 a and 6 b assembledtogether.

FIG. 8 a is a top view of the upper plate of FIG. 3 a, with thebelleville washer of FIGS. 5 a-5 c fitted within a retaining wall and aretaining ring of the upper plate.

FIG. 8 b is a top view of the lower plate of FIG. 3 b.

FIG. 9 is a cross-section view of an alternate embodiment of the presentinvention, which utilizes a belleville washer of the type shown in FIGS.5 a-5 c, showing the plates of FIGS. 8 a and 8 b assembled together.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be described more fully hereinafterwith reference to the accompanying drawings, in which particularembodiments and methods of implantation are shown, it is to beunderstood at the outset that persons skilled in the art may modify theinvention herein described while achieving the functions and results ofthis invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

Referring now to FIGS. 3 a and 3 b, side cross-section views of upperand lower plate members 100,200 of the preferred embodiment of thepresent invention are shown. As the device is designed to be positionedbetween the facing surfaces of adjacent vertebral bodies, the platesinclude substantially flat external face portions 102,202 which seatagainst the opposing bone surfaces. In addition, the plates are to matewith the bone surfaces in such a way as to not rotate relative thereto.It is, therefore, preferred that the external faces of the platesinclude a porous coating 104,204 into which the bone of the vertebralbody can grow. (Note that this limited fusion of the bone to the baseplate does not extend across the intervertebral space.) A hole (notshown) can be provided in the upper plate such that the interior of thedevice may be readily accessed if a need should arise.

The upper plate 100 includes an internal face 103 that includes anannular retaining wall 108 and an annular retaining ring 109. The lowerplate 200 includes an internal face 203 that includes a central postmember 201 which rises out of the internal face 203 at a nearlyperpendicular angle. The top of this post member 201 includes aball-shaped head 207. The head 207 includes a series of slots whichrender it compressible and expandable in correspondence with a radialpressure (or a radial component of a pressure applied thereto). The head207 includes a central threaded axial bore 209 which extends down thepost 201. This threaded bore 209 is designed to receive a set screw 205.Prior to the insertion of the set screw 205, the ball-shaped head 207 ofthe post 201 can deflect radially inward because of the slots (so thatthe ball-shaped head contracts). The insertion of the set screw 205eliminates the capacity for this deflection.

Referring now to FIGS. 4 a and 4 b, a spirally slotted belleville washer130 having radially spaced concentric grooves is provided in top andside cross-section views. The belleville washer 130 is a restoring forceproviding device which comprises a circular shape, having a centralopening 132, and which is radially arched in shape. The bellevillewasher 130 has a radial convexity 134 (i.e., the height of the washer130 is not linearly related to the radial distance, but may, forexample, be parabolic in shape). The restoring force of the bellevillewasher 130 is proportional to the elastic properties of the material.

The belleville washer 130 comprises a series of spiral slots 131 formedtherein. The slots 131 extend from the outer edge of the bellevillewasher, inward along arcs generally directed toward the center of theelement. The slots 131 do not extend fully to the center of the element.In preferred embodiments, the slots may extend anywhere from a quarterto three quarters of the overall radius of the washer, depending uponthe requirements of the patient, and the anatomical requirements of thedevice.

The belleville washer 130 further comprises a series of grooves 133formed therein. The grooves 133 are concentric and radially spaced fromthe outer edge of the belleville washer toward the center of theelement. Each groove 133 is defined on the surface of the bellevillewasher and has a depth and a width along the length of the groove. Thewidth and depth of each groove are defined by the surface of thebelleville washer including a bottom surface, right surface and leftsurface such that the right and left surfaces are adjacent to andtransverse to the bottom surface, as is illustrated in, for example,FIGS. 4 b, 5 b and 5 c. In the preferred embodiment shown in FIGS. 4 aand 4 b, the width 135 of each groove 133 is uniform along the length ofthe groove 133. Further in the preferred embodiment, the depth 137 ofeach groove 133 is uniform along the length of the groove 133. Furtherin the preferred embodiment, each groove 133 has a different widthconfiguration and a different depth configuration than each other groove133. More specifically, in the preferred embodiment, the width dimensionand the depth dimension both vary from groove to groove, each increasingincrementally from groove to adjacent groove with increasing distancefrom the center of the washer 130. Stated alternatively, grooves thatare relatively more narrow and more shallow than the other grooves arecloser to the center of the washer, whereas grooves that are relativelywider and deeper than the other grooves are closer to the outer edge ofthe washer. This is illustrated by example in FIGS. 4 a and 4 b, whichshow three concentric grooves 133 a-c, with the outermost groove 133 cbeing deeper and wider than groove 133 b, which is in turn deeper andwider than groove 133 a. Further in the preferred embodiment, the radialspacing of the grooves is uniform.

It should be understood that in other embodiments, one or both of thedepth and the width of each groove can be (1) increasing along thelength of the groove, (2) decreasing along the length of the groove, or(3) varied along the length of each groove, either randomly or accordingto a pattern. Moreover, in other embodiments, it can be the case thateach groove is not formed similarly to one or more other grooves, withor without respect to width and depth dimensions, but rather one or moregrooves are formed in any of the above-mentioned fashions, while one ormore other grooves are formed in another of the above-mentioned fashionsor other fashions. Also, in other embodiments, it can be the case thatthe radial distance between the grooves is not the same, but rather thespacing increases the closer the space is to the outer edge of thewasher, decreases the closer the space is to the outer edge of thewasher, or varies either randomly or according to a pattern. Also, whilethe grooves of the preferred embodiment and the illustrated alternateembodiment have lengths that form closed loops, it should be noted thatin other embodiments, the concentric grooves can have lengths that formopen loops or arcs; for example, a two concentric grooves forming openloops or arcs can be used in place of a single concentric groove forminga closed loop. It should be clear that any concentric groove pattern canbe implemented without departing from the scope of the presentinvention. To illustrate an alternate embodiment showing an alternateradially spaced concentric groove pattern, FIGS. 5 a-5 c show abelleville washer 130 having radially spaced concentric grooves 133 intop and side cross-section views, with each groove 133 having a widthand a depth each varying along the length of the groove 133, with eachgroove 133 being formed differently than at least one other groove 133,with the radial spacing of the grooves 133 being varied, and with bothclosed loops and open loops or arcs being used. In this alternateembodiment, the difference between the grooves 133 is characterized inthat the wider and deeper portion of any particular groove 133 is on adifferent side of the washer 130 than the wider and deeper portion of atleast one other groove 133.

As a compressive load is applied to the belleville washer 130 of thepresent invention, the forces are directed into a hoop stress whichtends to radially expand the washer. This hoop stress is counterbalancedby the material strength of the washer, and the force necessary to widenthe spiral slots 131 and the radially spaced concentric grooves 133along with the strain of the material causes a deflection in the heightof the washer. Stated equivalently, the belleville washer 130 respondsto a compressive load by deflecting compressively; the spiral slotsand/or radially spaced concentric grooves cause the washer to furtherrespond to the load by spreading as the slots and/or the grooves in thewasher expand under the load. The spring, therefore, provides arestoring force which is proportional to the elastic modulus of thematerial in a hoop stressed condition.

More particularly, the central opening 132 of the belleville washer isenlarged. This central opening 132 includes a curvate volume 233 forreceiving therein the ball-shaped head 207 of the post 201 of the lowerplate 200 described above. More particularly, the curvate volume 233 hasa substantially constant radius of curvature which is also substantiallyequivalent to the radius of the ball-shaped head 207 of the post 201. Inthis embodiment, the spiral slots 131 do not extend all the way to thecentral opening 132, and approach the opening only as far as thematerial strength of the washer can handle without plastically deformingunder the expected anatomical loading. Preferably, the center of thewasher is flat; therefore, the central opening 132 can be formed fromflat edges. It should be understood that this is not required, butrather is preferred.

Referring now to FIG. 6 a, a top view of the upper plate 100 of FIG. 3a, with the spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 4 a and 4 b fitted within a retaining wall 108 and aretaining ring 109 of the upper plate 100, is shown. The diameter of theretaining wall 108 is preferably slightly wider than the diameter of theundeflected belleville washer 130 such that the loading thereof canresult in an unrestrained radial deflection of the washer 130. FIG. 6 bshows a top view of the lower plate 200 of FIG. 3 b.

FIG. 7 shows the fully assembled preferred embodiment of the presentinvention. The spirally slotted and radially grooved belleville washer130 of FIGS. 4 a and 4 b is placed with its wide end against the topplate 100 within the annular retaining wall 108 as shown in FIG. 6 b.The annular retaining ring 109 is provided to hold the belleville washer130 against the internal face 103 of the upper plate 100 within theretaining wall 108. The post 201 of the lower plate 200 is fitted intothe central opening 132 of the belleville washer 130 (the deflectabilityof the ball-shaped head 207 of the post 201, prior to the insertion ofthe set screw 205, permits the head 207 to be inserted into the interiorvolume 233 at the center of the belleville washer 130. Subsequentintroduction of the set screw 205 into the axial bore 209 of the post201 eliminates the deflectability of the head 207 so that the washer 130cannot be readily removed therefrom, but can still rotate thereon. Insome embodiments (not in this preferred embodiment), the post head 207can be locked tightly within the central volume 233 of the bellevillewasher 130 by the tightening of the set screw 205, to prevent anyrotation of the plates 100,200. Compressive loading of the assemblycauses the washer 130 to deflect (with the spiral slots and the radiallyspaced concentric grooves enhancing the deflection) so that the wide endradially expands while being maintained centrally against the upperplate 100 by the retaining wall 108 and the retaining ring 109. When theload is removed, the washer 130 springs back to its original shape.

Referring now to FIG. 8 a, a top view of the upper plate 100 of FIG. 3a, with the spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 5 a-5 c fitted within a retaining wall 108 and aretaining ring 109 of the upper plate 100, is shown. The diameter of theretaining wall 108 is preferably slightly wider than the diameter of theundeflected belleville washer 130 such that the loading thereof canresult in an unrestrained radial deflection of the washer 130. FIG. 8 bshows a top view of the lower plate 200 of FIG. 3 b.

FIG. 9 shows a fully assembled alternate embodiment of the presentinvention. The spirally slotted and concentrically grooved bellevillewasher 130 of FIGS. 5 a-5 c is placed with its wide end against the topplate 100 within the annular retaining wall 108 as shown in FIG. 6 b.The annular retaining ring 109 is provided to hold the belleville washer130 against the internal face 103 of the upper plate 100 within theretaining wall 108. The post 201 of the lower plate 200 is fitted intothe central opening 132 of the belleville washer 130 (the deflectabilityof the ball-shaped head 207 of the post 201, prior to the insertion ofthe set screw 205, permits the head 207 to be inserted into the interiorvolume 233 at the center of the belleville washer 130, and the washer130 to be rotated into the desired angulation; subsequent introductionof the set screw 205 into the axial bore 209 of the post 201 eliminatesthe deflectability of the head 207 so that the washer 130 cannot bereadily removed therefrom, but can still rotate thereon.). The post head207 can be locked tightly within the central volume 233 of thebelleville washer 130 by the tightening of the set screw 205, to preventany rotation of the plates 100,200. Compressive loading of the assemblycauses the washer 130 to deflect (with the spiral slots and radiallyspaced concentric grooves enhancing the deflection) so that the wide endradially expands while being maintained centrally against the upperplate 100 by the retaining wall 108 and the retaining ring 109. When theload is removed, the washer 130 springs back to its original shape.

Inasmuch as the human body has a tendency to produce fibrous tissues inperceived voids, such as may be found within the interior of the presentinvention, and such fibrous tissues may interfere with the stable and/orpredicted functioning of the device, some embodiments of the presentinvention (although not the preferred embodiment) will be filled with ahighly resilient and biologically inert elastomeric material. Suitablematerials may include hydrophilic monomers such as are used in contactlenses. Alternative materials include silicone jellies and collagenssuch as have been used in cosmetic applications.

While there has been described and illustrated specific embodiments ofan intervertebral spacer device, it will be apparent to those skilled inthe art that variations and modifications are possible without deviatingfrom the broad spirit and principle of the present invention. Theinvention, therefore, shall not be limited to the specific embodimentsdiscussed herein.

1. An intervertebral spacer device comprising: first and second plates,each having inner and outer surfaces thereof, said plates being disposedin a spaced apart relationship such that the inner surfaces face towardone another, and the outer surfaces face away from one another, saidfirst plate including a retaining wall extending outwardly from saidinner surface of said first plate; and a belleville washer, havingnarrow and wide ends thereof, disposed with said wide end contactingsaid inner surface of said first plate within said retaining wall, suchthat a compressive load applied to the outer surfaces of said plates iscounteracted by said belleville washer, said belleville washer includinga plurality of radially spaced concentric grooves; wherein eachconcentric groove of said plurality of radially spaced concentricgrooves in said belleville washer has a respective length, a respectivedepth along said respective length, and a respective width along saidrespective length, at least one of said respective depth and saidrespective width being uniform along said respective length; whereineach of said plurality of radially spaced concentric grooves in saidbelleville washer is at a respective distance from an outer edge of saidbelleville washer, wherein said depths differ with respect to oneanother depending on said distances, and said widths differ with respectto one another depending on said distances.
 2. The device as claimed inclaim 1, wherein said second plate member comprises a post structurehaving a ball-shaped head and said belleville washer has a centralopening with a curvate volume for receiving and holding therein saidball-shaped head.
 3. The device as set forth in claim 1, wherein saidbelleville washer has a top surface, a bottom surface, and a thicknesstherebetween, and said respective depth of each of said plurality ofradially spaced concentric grooves is less than said thickness of saidbelleville washer.
 4. An intervertebral spacer device comprising: firstand second plates, each having inner and outer surfaces thereof, saidplates being disposed in a spaced apart relationship such that the innersurfaces face toward one another, and the outer surfaces face away fromone another, said first plate including a retaining wall extendingoutwardly from said inner surface of said first plate; and a bellevillewasher, having narrow and wide ends thereof, disposed with said wide endcontacting said inner surface of said first plate within said retainingwall, such that a compressive load applied to the outer surfaces of saidplates is counteracted by said belleville washer, said belleville washerincluding a plurality of radially spaced concentric grooves; whereineach concentric groove of said plurality of radially spaced concentricgrooves in said belleville washer has a respective length, a respectivedepth along said respective length, and a respective width along saidrespective length, at least one of said respective depth and saidrespective width being uniform along said respective length; whereineach of said plurality of radially spaced concentric grooves in saidbelleville washer is at a respective distance from an outer edge of saidbelleville washer, wherein said depths differ with respect to oneanother depending on said distances, and said widths differ with respectto one another depending on said distances, wherein a first radiallyspaced concentric groove located closer to an outer edge of saidbelleville washer has a greater depth than a second radially spacedconcentric groove located further away from the outer edge of saidbelleville washer than said first radially spaced concentric groove. 5.The device as claimed in claim 4, wherein said second plate membercomprises a post structure having a ball-shaped head and said bellevillewasher has a central opening with a curvate volume for receiving andholding therein said ball-shaped head.
 6. The device as set forth inclaim 4, wherein said belleville washer has a top surface, a bottomsurface, and a thickness therebetween, and said respective depth of eachof said plurality of radially spaced concentric grooves is less thansaid thickness of said belleville washer.
 7. An intervertebral spacerdevice comprising: first and second plates, each having inner and outersurfaces thereof, said plates being disposed in a spaced apartrelationship such that the inner surfaces face toward one another, andthe outer surfaces face away from one another, said first plateincluding a retaining wall extending outwardly from said inner surfaceof said first plate; and a belleville washer, having narrow and wideends thereof, disposed with said wide end contacting said inner surfaceof said first plate within said retaining wall, such that a compressiveload applied to the outer surfaces of said plates is counteracted bysaid belleville washer, said belleville washer including a plurality ofradially spaced concentric grooves; wherein each concentric groove ofsaid plurality of radially spaced concentric grooves in said bellevillewasher has a respective length, a respective depth along said respectivelength, and a respective width along said respective length, at leastone of said respective depth and said respective width being uniformalong said respective length; wherein each of said plurality of radiallyspaced concentric grooves in said belleville washer is at a respectivedistance from an outer edge of said belleville washer, wherein saiddepths differ with respect to one another depending on said distances,and said widths differ with respect to one another depending on saiddistances, wherein a first radially spaced concentric groove locatedcloser to an outer edge of said belleville washer has a greater widththan a second radially spaced concentric groove located further awayfrom the outer edge of said belleville washer than said first radiallyspaced concentric groove.
 8. The device as claimed in claim 7, whereinsaid second plate member comprises a post structure having a ball-shapedhead and said belleville washer has a central opening, with a curvatevolume for receiving and holding therein said ball-shaped head.
 9. Thedevice as set forth in claim 7, wherein said belleville washer has a topsurface, a bottom surface, and a thickness therebetween, and saidrespective depth of each of said plurality of radially spaced concentricgrooves is less than said thickness of said belleville washer.